1. Field of the Invention
This invention relates to the field of feed forward systems, and more particularly relates to feed forward servo systems in disk drives.
2. Description of the Related Art
Systems with repeatable runout (RRO) signals often use a feed forward servo system to cancel the position error caused by the RRO signals. RRO signals are predictable because of their periodic nature. RRO signals have many origins. In disk drives, imperfections in a disk assembly or the data storage media, misaligned or unbalanced disk platters, a tilt in the spindle motor, or eccentricities introduced into data tracks at servo write time may cause RRO signals. These or other defects may cause synchronized harmonic disturbances when the disk rotates. These defects are not related to track density, and therefore do not scale with the track density. This means that as track densities increase in modern disk drives, the RRO signals have more negative effects on the disk drive's ability to properly read, and write data from the disk. Methods that have been used in the past to cancel RRO signals are no longer fast enough or complete enough for modern track densities.
Both feed forward systems and feedback systems have been used to remove RRO signal components from input signals. Feed forward systems are also referred to as open loop systems. Feed forward systems typically comprise two or more signals added or subtracted to form an output. A cancellation signal is usually added or subtracted from another signal to eliminate some component of the other signal, like an RRO signal component, to maintain or achieve a specific system state. Feedback systems, conversely, are referred to as closed loop systems. In a feedback system, the output of the system is added or subtracted to the input signal, again to eliminate some component, like an RRO signal component, from the input signal to maintain or achieve a specific system state.
Feed forward systems can more completely cancel disturbances that are known and measurable than feedback systems can, but do not react well to novel disturbances. Feedback systems can cancel novel disturbances, but do not cancel the disturbance until the output of the system has already been affected. Feed forward systems also have a lower required sampling frequency than do feedback systems. This makes it possible to build an inexpensive large scale integration (LSI) chip. For example, a feed forward system with a sampling frequency of 2 kHz can have a cancelling effect similar to a feedback system with a sampling frequency of 20 kHz. The gain of either system must be carefully set to avoid instability or oscillation due to overcorrection. While methods are well known in the art for determining an optimal gain for a feedback system, feed forward systems depend primarily on trial and error to determine the optimal gain. If the gain is too low, the feedback system is very slow. If the gain is too high, the system becomes unstable.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method to remove RRO components from an input signal using a combined feed forward and feedback system that react as quickly and precisely as possible without going unstable. Beneficially, such an apparatus, system, and method would have an optimal gain for quick reaction to disturbances, without instability from overcorrecting.